An electrophotographic image forming apparatus includes a developing device having a developer bearing member which brings developer, such as toner particles, onto an electrostatic latent image bearing member for development and a toner supply roller which is disposed in contact with the developer bearing member and supplies toner particles to and collects them from the developer bearing member at the contact area thereof.
The United States Patent Application No. 2001/0036376 A1 discloses the toner supply roller which includes a core bar and a form layer disposed around the circumference of the core bar. The circumferential layer is made of resin foam, such as urethane foam or rubber foam, which can cause certain disadvantages due to the property of the material. For example, the foam layer with a lower air permeability is incapable of releasing toner particles accommodated therein. The unreleased accommodated toner particles tend to cause an aggregation within the foam layer, which loses its elasticity and thereby increases a frictional contact against the developer bearing member and also toner particles borne thereon. This results in an unwanted adhering of the toner particles on the resultant images.
The foam layer with a higher air permeability, on the other hand, has a less scraping ability for scraping toner particles from the developer bearing member. The low scraping ability results in that the toner particles on the developer bearing member is unlikely to be replaced by another toner particles. This deteriorates the toner particles on the developer bearing member. Also, this causes the toner particles to be forced onto the surface of the developer bearing member, forming an unwanted toner film, i.e., so-called “filming”, on the developer bearing member.
The foam layer with a lower hardness is incapable of strongly forcing toner particles onto the developer bearing member, which provides a less amount of toner onto the developer bearing member and therefore causes images with insufficient densities. In addition, the lower hardness of the foam layer provides insufficient scraping of the developer bearing member, which causes the unwanted filming.
The foam layer with a higher hardness tends to be strongly forced against the developer bearing member, which in turn damages the toner particles, such as cracking, and also forces external additives into the surface of the toner particle, i.e., unwanted implantation of the additives into toner particles.
The foam layer with a lower average cell-density makes fewer contacts with the developer bearing member, which exercises a low scraping ability of toner and thereby results in the deterioration of the toner particles and the unwanted filming on the developer bearing member.
The foam layer with a higher average cell-density makes a larger number of contacts with the developer bearing member, which tends to damage the toner particles.
As discussed above, the foam layer of the supply roller needs appropriate air permeability, hardness, and average cell-density. Therefore, the present invention is to provide suitable properties to the toner supply roller, thereby preventing the generation of filming and then allowing the image forming apparatus to produce high quality images free of unwanted toner